[compiler-rt] [compiler-rt] Add COMPILER_RT_GPU_THIN_LTO to build with -flto=thin (PR #226916)

[Wenju He via llvm-commits]

wenju-he wrote:

> A side question: does Intel support object linking in downstream? AFAIK ThinLTO requires object linking to work.

Yes, we support object linking (including rdc cross-TU symbols) in downstream, but that is done in the NEO runtime, not in compiler.
I think lld should also work because our binary is ELF format and I had a POC for that in AOT mode, but the problem is that there won't be a PISA (like PTX) linker, so JIT support will need additional work (coordinated change between compiler and runtime). Therefore, compiler support for binary linking is not in plan yet.

https://github.com/llvm/llvm-project/pull/226916